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From the available observational and theoretical evidence it is argued that the mass of molecular gas in the Galaxy has been considerably overestimated and that the ages of the giant molecular complexes do not exceed a few times 10-million years. An expression derived for the collisional time scale for clumps in a complex has a maximum value of 10-million yr. It is argued that the formation of giant complexes by random collisional agglomeration of small molecular clouds is incompatible with several firm observational results. The Parker instability is discussed as a possible formation mechanism which can explain many of the observed properties of the complexes.
Blitz et al. (Thu,) studied this question.